The angiosperm ovule plays a central role in producing and sheltering the female gametophyte which ultimately gives rise to the egg cell in plant sexual reproduction (Bouman, In Embryology of Angiosperms, B. M. Johri, eds (Berlin, Germany: Springer-Verlag), pp. 123-157 (1984)). Additionally, the final product of the ovule, the seed, is a highly significant human food source as well as a common means of propagation of species in which other plant organs are agriculturally important.
As a result of its importance, there is a wealth of descriptive knowledge concerning ovule anatomy and morphology (see, e.g., Huang and Russell, Int. Rev. Cytol. 140:233-293 (1992)), though little is known about the molecular basis of ovule development and function. The relative inaccessibility of the ovule within the ovary, as well as the difficulty in harvesting adequate amounts of tissue at known developmental stages, has impeded progress in understanding the molecular basis of ovule development and function. Recently, however, several groups have taken genetic approaches to dissect the underlying mechanisms involved in ovule development. Several mutations affecting ovule and female gametophyte development have been identified, including bel1, sin1, ovm2 and ovm3, which cause female sterility in Arabidopsis (Robinson-Beers et al., Plant Cell 4:1237-1249 (1992); Reiser and Fischer, Plant Cell 5:1291-1301 (1993); Modrusan et al., Plant Cell 6:333-349 (1994)). Other genes recently shown to be involved in ovule development are ats (Leon-Kloosterziel et al., Plant Cell 6:385-392 (1994)) and sup (Gaiser et al., Plant Cell 7:333-345 (1995)). A few mutations defective in aspects of megasporogenesis have also been identified, such as msg in wheat (Joppa et al., 1987), sy-2 in Solanum (Parrott and Hanneman Jr., Dun. Genome 30:536-539 (1988)) and Gf in Arabidopsis (Redei, Genetics 51:857-872 (1965)).
In spite of the recent progress in defining the genetic control of ovule development, little progress has been reported in the identification and analysis of genes expressed specifically in the ovule and embryo sac. Characterization of such genes would allow for the genetic engineering plants with a variety of desirable traits. For instance, inhibition of genes associated with ovule development may be used to produce seedless fruit. Alternatively, promoters from ovule specific genes can be used to direct expression of desirable heterologous genes to the ovule. The present invention addresses these and other needs.